A typical process for manufacturing semiconductor IC devices may include more than one hundred process steps, which are usually classified into the following categories: diffusion; photolithography; etching; ion implantation; deposition; and sputtering. Among these categories, processes such as diffusion or deposition generally require a long process time to perform. Thus, these processes having long process times are commonly performed on several wafer lots (commonly referred to as a batch). For example, a typical diffusion process step takes four to twelve hours for simultaneously processing a batch of up to six lots.
Scheduling policy thus becomes an important task in managing a production system. For example, a scheduling policy may be optimized to maximize throughput of wafers out of the production system or factory; minimize the average time wafers spend in the factory to reduce the amount of work-in-progress (WIP); or maximize the utilization of machines (also referred to as tools). One aspect of production scheduling is controlling the release of pending jobs onto the processing floor (i.e., lot release). Another aspect of production scheduling is lot dispatching, which deals with the control of jobs already on the processing floor. More specifically, lot dispatching concerns scheduling which lot, or lots, of the WIP are to be processed when a machine becomes available.
In a typical, conventional multiple-step workstation, such as a workstation used in an integrated circuit (IC) fabrication plant, the throughput largely depends on the cycle time for lots waiting to be processed in a batch sequential-type machine. More specifically, the throughput is substantially related to delay time. Delay time is the time the present lot is waiting to be processed but, due to the preceding lot being processed, the present lot is not being processed or is not in transit between process steps. In this conventional method, the operator relies on his or her experience and judgment to decide which order is to be taken for dispatching the lots. This subjective method often results in inefficient use of the processing machines.
Accordingly, there is an unmet need in the art for a method for use in manufacturing semiconductor integrated circuit devices that maximizes the throughput of a multiple-step workstation.